Duskmage (3.5e Feat)
From D&D Wiki
Combines Duskmage/ other arcane spellcaster abilites
Prerequisite: 2nd Level Arcane Spells, 3+ Level Duskblade
Benefit: When you take this feat, you must choose an arcane spell casting class which you can cast 2nd level spells in. From this point on, this class stacks with Duskblade for determining the caster level of both classes, to a maximum of the HD of the character. The character may also cast spells from this class and ignore the arcane spell failure chance due to wearing light armor and/or using a light shield so long as the level of the spell to be cast is no greater than your Duskblade class level plus one. If your other arcane class is Wizard, or another class where you prepare your spells, you may prepare Duskblade spells that are on your second class's spell list as if you had 'Spell Mastery' with them, but you are not considered to have 'Spell Mastery' for the spell for the purpose of taking feats or other abilities.
Normal: You suffer spell failure when casting arcane spells in armor. You may not prepare spells which you know in one class using spell slots from another class.